Examples of the industrially utilized reverse osmosis membranes include Loeb type membranes described in U.S. Pat. Nos. 3,133,132 and 3,133,137 as asymmetric membranes made of cellulose acetate. On the other hand, reverse osmosis composite membranes, in which an active thin film substantially having a selective separation property is formed on a microporous support film, are known as reverse osmosis membranes having a different structure from the asymmetric reverse osmosis membranes.
Presently, a number of such reverse osmosis composite membranes, in which a thin film of polyamide obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional aromatic acid halide is formed on a support film are known (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 55-147106, Publication of Japanese Patent Application (Tokkai Sho) No. 62-121603, Publication of Japanese Patent Application (Tokkai Sho) No. 63-218208, and Publication of Japanese Patent Application (Tokkai Hei) No. 2-187135). Also, those having a thin film of polyamide obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional alicyclic acid halide formed on a support film are known (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 61-42308).
In addition, various methods for after treatment of the reverse osmosis membrane are disclosed. For example, methods using various polymers as a protective layer are disclosed (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 51-13388, Publication of Japanese Patent Application (Tokkai Sho) No. 53-16372, Publication of Japanese Patent Application (Tokkai Sho) No. 62-197105, and Publication of Japanese Patent Application (Tokko Hei) No. 7-90152).
Recently, it has been expected to apply a reverse osmosis membrane to a treatment for water containing fouling substances such as various surfactants, for example, sewage. In addition to the high performance of the reverse osmosis membrane (a high salt rejection and a high water permeability), a high fouling resistance is required to maintain the desired flux for a long period. The above reverse osmosis membranes and the conventional after treatment methods are not sufficient to satisfy these two requirements. Therefore, a reverse osmosis composite membrane having a higher performance has been sought.
One fouling mechanism includes the charge condition of the membrane. For example, the surface of a cross-linked polyamide reverse osmosis membrane obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional alicyclic acid halide has a negative charge due to the residual carboxylic acid. The membrane surface having negative charge adsorbs, for example, cationic fouling substances, decreasing the flux. Therefore, a membrane has been required that is neutral in charge and has a high water permeability and a high salt rejection.